1. Field of the Invention
The present invention relates to a lubricant composition suited to lubricate an expandable shaft comprising a pair of shaft members axially expandable and contractible and coupled to each other to be integrally rotatable in a direction of rotation around its axis to fill an area between both the shaft members, the expandable shaft, and a steering system incorporating the expandable shaft.
2. Description of Related Art
Examples of EPS (Electric Power Steering System) for automobiles include column-type EPS utilizing a rotary force of an electric motor provided on a column. In the column-type EPS, an expandable shaft comprising a pair of shaft members axially expandable and contractible and coupled to each other to be integrally rotatable in a direction of rotation around its axis has spread as an intermediate shaft for connecting a steering shaft connected to a steering wheel serving as a steering member and a steering mechanism such as a rack-and-pinion mechanism.
In the column-type EPS, the distance between the steering shaft and the steering mechanism is arbitrarily changed when the expandable shaft is assembled into an automobile utilizing an expanding/contracting function of the expandable shaft, to improve assembling workability and absorb relative displacement between the steering shaft and the steering mechanism when the automobile is traveling.
As a coupling structure of the expandable shaft, a spline or a serration is generally employed. For example, the expandable shaft is configured by forming a male spline (external spline) at an end of one of the pair of shaft members (inner shaft member) and forming a cylindrical female spline (internal spline) into which the male spline is inserted at an end of the other shaft member (outer shaft member), and inserting the male spline into the female spline to couple both the shaft members to be axially expandable and contractible and integrally rotatable in the direction of rotation around the axis.
In this case, the expandable shaft cannot be assembled by inserting the male spline into the female spline if there is no clearance between the splines. When a clearance is provided, as disclosed in paragraphs [0002] to [0005] in Japanese Unexamined Patent Publication No. JP2001-50293A, however, both the shaft members are relatively displaced in a radial direction and a circumferential direction to produce a rattle sound, which makes a driver uncomfortable.
Therefore, a surface of at least one of both the splines is subjected to resin coating to reduce the clearance as small as possible, and the clearance is filled with a lubricant composition such as grease. In recent years as the column-type EPS has found wider applications to cars of large displacement, and, a load (torsional rigidity) applied to the steering column has tended to increase at the time of steering. Therefore, a higher surface pressure than before is applied between both the splines. If normal grease is used for lubricating the expandable shaft, stick-slip due to oil film shortage occurs, which causes a new problem of making a driver uncomfortable, by a slip sound easily produced particularly when the steering wheel is returned.